fix: bugs found during fuzzing of xrow_decode_raft

Summary

Fixes several bugs found during fuzzing of xrow_decode_raft. Previously fuzzing could not find those due to an incorrect test setup.

• test/fuzz: update xrow_decode_raft corpus after test fix for better coverage
• fix: missing check for inconsistent vclock values in mp_decode_vclock_ignore0
• fix: missing check for mp type in xrow_decode_raft

xrow.c fuzzing coverage +4%

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• Relates to https://git.picodata.io/picodata/picodata/picodata/-/issues/461
• Docs follow-up: not necessary